Well, the left has a team advantage since they were part of the Round Table. With the exception of Shielder. Sort of.
Left team has no Master (unless Merlin can serve as one, which limits his own magical power), meanwhile the right side has Arjuna and Scathach (who can serve as her own Master and can even summon other Heroic Spirits). They also have Mysterious Heroine X, who does extra damage to Saber-faces, which the left side has two of.
Left side's big advantage is that FeMC is helpless on her own, so they outnumber the fighters on the right side (or at least they do until Scathach summons her own Servants), plus Merlin is a complete unknown.
Assuming a real fight with no limits from game mechanics; either Merlin does something big to take out right side's heavy hitters or Arjuna and Scathach unleash their big moves and take out two or three of the left side's fighters right off the bat (combined, Pashupata and Gae Bolg Alternative might even take out everyone but Merlin).